1. Field of the Invention
A variable output induction motor drive system for use with a polyphase variable output alternating current motor.
2. Description of the Prior Art
Induction motors are the most widely used source of electric drive power in industrial and domestic applications. Their popularity is due to the basic simplicity and ruggedness of the basic concept, and relatively low cost manufacture. Induction motors do not use brushes or slip rings, and have the fewest windings and the least insulation requirements compared with other types of DC or AC motors.
However, the induction motor has one major limitation in applications requiring variable speed operation because the number of magnetic poles in the motor and the frequency of the power source determine the speed of rotation. In the United States where the power line frequency is 60 Hz, a two-pole induction motor therefore runs at a nominal 3600 rpm. Unfortunately, such a constant speed motor is not well suited in many industrial processes where variable throughput is required. Until variable speed drives became available for induction motors, one solution was to apply mechanical throttling at the output with the motor running at normal speed. This resulted in an unacceptably wasteful use of power to control throughput. About 25 years ago, the development of low cost semiconductor power devices opened up the possibility of designing cost effective and efficient variable frequency power sources capable of driving variable speed drive systems for three phase induction motors.
However, controlling the speed of an induction motor is more difficult than controlling the speed of a DC motor in which the torque is basically proportional to the product of the flux per pole and the armature current.
In a DC motor, separate connections to the field and the armature windings are generally available so that the field winding may be connected either in series or in parallel with the armature winding, or separately excited. Various well-known techniques have been developed to enable the torque and the speed of a DC motor to be controlled over a wide range. For instance, with separate excitation of the windings, adjustable speed control is obtained by operating with a fixed field flux and varing the armature voltage. The no load speed is determined as the speed where the induced voltage is equal to the applied voltage.
Adjustable torque operation is obtained in a separately excited DC machine by controlling the value of the armature current. With a constant value of field flux, the torque is then directly proportional to the value of the armature current. If the armature current is provided from a current source, the torque can be adjusted as accurately and as rapidly as the armature current can be adjusted and controlled. On the other hand, in a three-phase Y connected AC induction motor, there are only four input connections, one to each stator winding and a neutral connection. The stator winding is connected to the supply and the polyphase currents circulating through the stator winding produce a magnetic field that rotates at synchronous speed. In a three-phase xe2x80x9csqurrel-cagexe2x80x9d motor, the rotor consists of a number of copper bars with their ends connected to stout copper end rings, causing them to be permanently short circuited on themselves. The lines of force of the stator field cut the rotor conductors to induce current causing the rotor to follow the stator field. The rotor winding is not directly accessible and rotor current is produced by induction rather than by a separately controlled source.
At no load, losses in the motor cause the rotor speed to be slightly less than the synchronous speed of the stator field. When a load is applied, the rotor speed slips behind the synchronous speed developing torque as a function of the difference in speeds. This difference is defined as xe2x80x9cabsolute slipxe2x80x9d. Another useful measure of slip is xe2x80x9cfractional slipxe2x80x9d, defined as the absolute slip divided by the speed of the stator field. The frequency of the rotor currents is then the synchronous speed of the stator field (the line frequency) multiplied by the fractional slip. Slip may also be measured as a percentage. A motor operating with a slip of 0.02 may therefore be referred to as having 2% slip. In a typical induction motor, full load slip can vary between about 1% in high power motors (10 HP to 100 HP), up to 5% in fractional HP models.
The breakdown torque level represents the maximum torque available from the motor and any further increase in load can not be met by increases in slip. In normal operation with a line frequency voltage source, the full load operating torque of an induction motor is generally limited to about 50% of the breakdown torque to allow for reasonable variations in load. Since the speed of the motor is a function of the line frequency applied to the stator windings, a basic variable speed motor drive system requires a variable frequency power source. In addition, because a constant amplitude air gap flux provides optimum operating conditions for the motor, the amplitude of the input voltages applied to the stator windings should vary linearly with frequency to provide constant V/Hz operation. This technique is widely used in general-purpose applications where fast response time and rapid speed changes are not required. In these simple variable-speed systems, an inverter having an output that is controlled in both frequency and voltage normally provides the variable-frequency drive power required by the motor.
Two types of inverter are widely used in general purpose drives, the six-step inverter and the pulse-width modulated inverter. The six-step inverter typically uses six semiconductor switches in a bridge arrangement. The three-phase line voltage is full wave rectified to produce a DC voltage across a smoothing capacitor. Regulation of the voltage across the smoothing capacitor can be obtained by replacing the input rectifiers with phase controlled SCRS. In this way, the amplitude of the six-step output voltage applied to the motor can be controlled in proportion to the output frequency of the inverter. Gating on IGBT switches in the proper sequence produces the six-step line-to-neutral voltages. This amplitude of the waveforms increases as the frequency is increased. However, the performance of six-step motor drive systems becomes unsatisfactory at slow speeds, e.g. below 5 Hz, due to noticeable torque pulsations that prevent the smooth generation of power.
On the other hand, PWM inverters can simulate sine wave voltages more effectively and produce smooth variations in torque at the slower speeds. These PWM inverters typically employ variable duration high frequency voltage pulses having repetition frequencies between 5 kHz and 20 kHz. The switching command signals for producing the modulated pulses can be generated by comparing a sinusoidal waveform with a high frequency triangular waveform.
In order to improve the motor response performances, many newer PWM designs are using repetition frequencies above 10 kHz. However, these high frequencies have been shown to introduce serious problems in many applications. High frequency pulse currents, generated by the fast rise and fall times of the applied rectangular voltage waveforms, circulate in the motor and can cause break-down in the lubricating oil films, causing seizure of the rotor bearings. Fast switching voltage waveforms produced by pulse-width modulation can also cause corona breakdown in the insulation of the stator windings, and can create unacceptable levels of radiated and conducted EMI. At high power levels, the lengths of the connecting cables between the inverter and the motor are severely limited due to the reflections and distortions produced by the PWM waveforms. These problems do not exist with sinusoidal motor input waveforms.
A major objective of this invention is to demonstrate how true sine wave currents can replace pulse-width modulated sources in variable speed drive applications to improve the performance of the motor without affecting the motor life or producing objectionable levels of EMI.
In general purpose V/Hz variable speed drives, load variations on the motor are met by the xe2x80x98slipxe2x80x99 produced in the difference between the rotor and stator speeds. A change in output loading produces a change in slip that provides the required increase in output torque. When the motor is operated from a conventional voltage source, a sudden increase in the mechanical load results in a requirement for a fast increase in the rotor current to supply the increased torque. A delay in the rise of the rotor current can cause the breakdown slip to be exceeded thereby producing loss of control.
The problem of poor transient response time in general purpose (V/Hz) drives is widely recognized and has resulted in the development of complex motor control systems, variously called field oriented control or vector control. These systems control the flux producing and torque producing components of the stator currents by modulating the three phase voltage inputs applied to the stator windings through the pulse-width modulation. By using increasingly complex digital processing, substantially improved dynamic performance has been obtained. While vector control can provide fast control over a wide speed range, some important control problems still remain at slow speeds due to difficulties in determining the value of rotor resistance that can vary over a wide range due to rotor heating during operation.
In the present invention, fast response time of the motor is obtained by the direct control of the input currents to the motor rather than by the control of input voltages. Three phase sine wave currents are generated in series resonant inverters operating at a nominal repetition frequency of 25 kHz. However, the current output to each phase winding is generated by two rectified sine wave currents per inverter cycle, thereby producing typical ripple frequencies of 100 kHz. Since the modulated output frequency range of the current source inverters is determined by the speed range of the motor, this covers from DC at locked rotor to about 220 Hz for the stator frequency range, while the absolute slip frequency typically varies from zero to 6 Hz. Therefore, in the present invention, even at maximum speed, there are more than 400 inverter current packets during one cycle of the stator current waveform.
The operation of an induction motor is often compared with that of a transformer where the stator winding is regarded as the primary and the rotor winding becomes the secondary. Normally, transformers are used with voltage sources to step-up or step-down the voltage or to provide isolation between the primary and secondary circuits. However, in certain cases, transformers are used in series circuits where power is generated as a current source such as in a series resonant LC inverter circuit and the load current is produced in the secondary windings. In these cases where the primary winding is in series with the current source, the secondary circuits must be loaded, otherwise the primary appears as an open circuit. Similarly, if a short is placed across the secondary, the short is transferred to the primary and appears across the stator windings.
In an induction motor, the apparent rotor resistance varies with changes in speed, while in a transformer, the resistance of the secondary winding remains essentially constant. Since the rotor is essentially a shorted turn, the step-down ratio in the motor is directly related to the number of turns on its stator winding. When the rotor is at rest, its apparent rotor resistance reflected into the stator is extremely small even through its value is multiplied by the square of the turns-ratio. Under these locked rotor conditions, the fractional slip is unity and the input frequency of the supply must be less than the breakdown slip. When using a voltage source under locked rotor conditions, the magnitude of the applied stator voltage must also be limited to prevent exceeding the maximum permitted input current. Since the output torque generated by the motor at a given slip frequency is a function of the square of the rotor current and the value of its rotor resistance, it is more effective and safer to operate with a current source as described in this invention rather than to use a voltage source as in conventional variable speed drive systems. In fact, it will be shown that there are several advantages in operating the motor with a sine wave current source over the full range from locked rotor to maximum speed.
The sine wave current used in the present invention may be described as a current packet inverter since the invention uses variable size packets of current produced in series resonant inverters to modulate the current applied to the motor.
The control system allows the value of slip to be controlled over a wide load and speed range. Choosing the optimum value of slip is a compromise between operating with a high power factor and operating at the most efficient points on the torque/slip characteristic. To minimize the motor current for a particular value of torque, it is desirable to set the slip for operation close to the peak of the characteristic. When a family of torque/slip curves for different values of motor current are constructed, the optimum value of slip can be determined for a particular level of output torque.
When the application calls for control of torque, as in a motor vehicle drive system, there are several major advantages in using a sine wave current source rather than the pulse-width modulated voltage sources used in most vehicle applications. These include improved low speed performance because variations in rotor resistance no longer affect the value of rotor current, longer bearing and insulation life and less EMI interference compared with conventional PWM systems. By setting the value of slip in a current source drive system, output torque may be controlled by adjustment of input current.
Several examples of efforts to satisfy these requirements are described in the patents discussed hereafter. However, as is apparent, none of these teaches or suggests the improved power supply control of the present invention.
U.S. Pat. No. 5,684,678 shows a LC resonant circuit for a resonant converter including a resonant capacitor and an inductor coupled to a fixed frequency AC supply. A DC current to vary the inductance controls an inductor in parallel with the resonant capacitor. The DC current in the control winding produces core fluxes which effects core permeability. The controlled inductor has the effect of changing the capacitor impedance and thus influences the converter output.
U.S. Pat. No. 5,617,308 teaches a resonant link inverter control for driving a multi-phase induction motor through three feed lines each connected to opposite sides of a DC bus by a pair of switches. A resonant tank circuit connects a source of DC voltage through an inductor to the DC bus and provides a tank capacitor in parallel with the DC bus. A bipolar transistor having an anti-parallel diode, which is in series with a clamp capacitor to form a clamp circuit connected across the tank inductance, is turned off in response to the clamp current bearing a predetermined relationship to voltage across the clamp circuit as well as voltage across the DC source. The current in each feed line is sampled in response to clamp current, which is just slightly less than that which causes turnoff of the clamp transistor.
U.S. Pat. No. 4,805,081 shows an inverter system having two resonating current sources, which are resonant at the same frequency capable of being combined. At low power levels the currents are substantially out of phase and the frequency is adjusted. At intermediate power levels the currents are adjusted in phase and the frequency is fixed. At high power levels the currents are substantially in phase and the frequency is adjusted.
U.S. Pat. No. 5,689,164 discloses an inductor-capacitor series resonant circuit connected in series with an unidirectionally conductive semiconductor switch to form a resonant network, one resonant network for each stator phase winding of a switched reluctance motor to be connected in parallel to the stator phase winding. Zero current switching is achieved by selecting the resonant frequency such that the inductor and capacitor resonate in a time period during both turn-on and turn-off of the semiconductor switches.
U.S. Pat. No. 4,998,054 relates to a DC link resonant converter for controlling the speed of AC machines. Switch and power supply means is provided to establish a bi-directional initial current in the resonant circuit. By selecting the plurality and magnitude of the initial current, the peak voltage of the resonant link is controlled and reliable zero crossing of the resonant voltage is assured.
U.S. Pat. No. 5,777,459 teaches an induction electrical power generation system for generating alternating current varying within a frequency range comprising an induction electrical generator for generating alternating current having a rotor and stator. The stator includes at least one phase winding to output the generated alternating current and a plurality of poles with at least two different numbers of poles within the plurality of poles being selected to generate the alternating current. An exciter-winding wound on the stator is driven by AC excitation, which varies in frequency during generation of the alternating current and the rotor operating with slip during the generation of the alternating current. A variable speed drive is coupled to the rotor for driving the rotor in a speed range during generation of the alternating current. A controller is coupled to the inverter and responsive to the rotor speed signal for commanding variation of the excitation frequency as a function of the rotor speed signal.
U.S. Pat. No. 5,734,250 shows a device for controlling a three-phase induction motor driven by an inverter connection. A full-wave rectified grid alternating voltage supplied to the inverter is arranged to be supplied to respective motor phase through an electronic switch connection such that respective xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods are determined by the drive frequency which is generated by an oscillator device.
U.S. Pat. No. 5,668,707 and U.S. Pat. No. 5,587,892 relate to a multi-phase AC to DC harmonic neutralizing power converter comprising a plurality of non-isolated inputs for respective phases of a multi-phase source of AC power and a plurality of first rectifiers connected respectively to the inputs. A multi-phase harmonic neutralizing converter includes a power switching inverter including LC resonant circuits having an input connected to the outputs of each of the first rectifiers. A plurality of second rectifiers connected to the output of the inverter through a plurality of respective transformers are connected in voltage additive relationship to the outputs of the respective individual phase first rectifiers.
The following patents are additional examples of such prior art: U.S. Pat. No. 4,843,296; U.S. Pat. No. 4,999,561; U.S. Pat. No. 5,280,421; U.S. Pat. No. 5,371,668; U.S. Pat. No. 5,440,219 and U.S. Pat. No. 5,629,598.
Extensive development efforts have been undertaken over the past twenty years to improve the inherently slow response times inherent in V/Hz variable speed AC motor drives. These have included vector control techniques that maintain the phase of the air-gap flux linkages in the motor while controlling the magnitude and frequency of the stator and rotor currents. In essence, vector control systems take the speed and position information of the rotor from a transducer, and use pulse width modulated inverters to control the magnitude, frequencies and the phases of the currents applied to stator windings to produce the desired amount of output torque. This is a complex process requiring the application of space vectors, coordinate transformations and complex machine models.
Additional complications are introduced by the use of pulse width modulation to synthesize the three phase sine waves applied to the motor since the timing of the switching frequency in the inverter must also be taken into consideration. The fast rise time pulses applied to the motor from the pulse width modulator can also cause failures in the motor bearings, insulation breakdown in the stator windings and produce EMI radiation into adjacent equipment. Remote operation between the inverter and the motor can be severely limited due to distortion of the inverter pulses in the inductance of the connecting cables. The problems generally associated with variable speed drive systems suggest that a different approach should be developed to provide the three-phase input power to the motor. Such an approach is the subject of the current source system described in this application. The inherent problems and constraints that are evident with V/Hz and vector control systems do not exist with current source sine wave drive systems that operate effectively at all power levels and do not produce high frequency pulse components that can cause damage to the motor. Control at slow speeds is unaffected by variations in stator winding resistance.
Pulse width modulation is not required in current source control systems. Fast rise time voltage pulses are replaced with true sine wave input currents, thereby eliminating EMI interference problems, and preventing motor damage caused by the input pulses. Also, the motors run cooler with sine wave inputs.
The instant control system allows independent adjustments of the stator current and slip frequency in an induction machine. In this way, direct control of torque is obtained by applying the data from a family of curves depicting torque versus slip frequency characteristics for different values of stator current. These curves may be derived from tests performed on an actual motor or from data supplied by the motor manufacturer.
The present invention relates to a variable output induction motor drive system for use with a polyphase variable output alternating current motor. Instead of using six step or pulse-width modulation to synthesize a sine wave voltage in a V/Hz or vector control drive, actual sine waves of current are fed to the polyphase variable output alternating current motor for the efficient control of speed and torque throughout the operating range of the polyphase variable output alternating curent motor from standstill to maximum speed.
Alternatively, in general purpose applications that use simple V/Hz control systems, actual sine waves of voltage can be generated by using a voltage feedback control instead of current feedback to control the power inverters.
The use of sine wave modulation instead of pulse-width modulation to drive the polyphase variable output alternating current motor eliminates many problems created by high frequency PWM rectangular pulses. Sine waves do not contain the fast rise time transient currents or voltages inherent in pulse-width modulation that produce excessive EMI interference in adjacent equipment or cause bearing and insulation failures in the motor.
The variable output induction motor drive system of the present invention employs a series resonant power inverter arrangement and a reverse power control that effectively provide four quadrant operation essential to effectively drive the input impedance of an induction motor that produces a phase current lag as a function of the motor power factor. This phase relationship causes a polarity shift between the motor current and voltage during a portion of each cycle. The series resonant power inverter arrangement supplies the sine wave power to the polyphase variable output alternating current motor over the major portions of the sine wave power cycle when the voltage and current have the same polarity. The reverse power control circuit supplies the motor current during that portion of the sine wave power cycle when the voltage and current are of opposite polarity.
The power factor of an induction motor under normal operating conditions typically varies between about 0.75 and about 1.0 depending on the slip and the load conditions of the motor. The reason the motor power factor is not constant is because its value depends upon the ratio of the input reactance to the input resistance. The input reactance varies directly with input frequency and the input resistance varies according to the ratio of the input frequency to the slip frequency. This means that the effective resistance of the rotor increases as the slip is reduced. Control circuits modulate the power source output currents both from the series resonant inverter arrangement and the reverse power control circuit to produce sine wave outputs at frequencies from about a few Hz at locked rotor to more than 220 Hz when switching at frequencies from about 15 kHz to about 50 kHz. A three phase system is programmed to produce variable frequency and variable amplitude sine waves of current or voltage suitable for driving a three phase variable speed induction motor. Because the sine wave generators, amplitude controllers and power amplifiers are directly coupled, it is also possible to apply controlled values of DC current to the motor to provide moderate dynamic braking.
The control system is capable of controlling the magnitude of slip generated in the polyphase variable output alternating current motor. This allows the polyphase variable output alternating current motor to operate under efficient conditions and by holding the slip at a substantially constant value, the torque may be directly controlled by adjusting the magnitude of the input current to the polyphase variable output alternating current motor.
In summary, the instant invention contemplates three variable output power inverters each with a reverse power control circuit including modulation to produce sine wave outputs of current with independent control of amplitude and frequency required for optimum performance of induction motors. The power source inverters use series resonant circuits to produce two high frequency sine wave current sources, typically operating at 25 kHz, that are subtracted or added to control the resulting output power from zero to maximum, at efficiencies greater than 90%. The series resonant and reverse power control or circuits normally produce a current source using a current feedback signal from the polyphase variable output alternating current motor. The present invention can also provide a voltage source by using feedback of the output voltage from the polyphase variable output alternating current motor.
An output transformer used in each inverter power stage may include taps that can be selectively switched to provide an optimum match between the power source and the polyphase variable output alternating current motor. By changing the transformer turns ratio, more efficient transfer of power is possible over a wide speed range, effectively providing an electronic gearbox that generates higher torque at slow speeds and higher output voltages at high speeds where field weakening is normally experienced.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.